This invention relates generally to electroforming, and more specifically to an apparatus and method for electrodepositing electroforms without internal stress.
Electroforming is a variation of electroplating involving the formation of a removable layer of metal which conforms exactly to the shape of the surface of a master. A primary problem in making electroforms is that internal stresses, usually tensile, are created in the deposit during electrodeposition. The internal stresses can cause deformation both during electrodeposition and after parting from the master. Many attempted advances in the electroforming art have been directed toward minimizing internal stresses in the electrodeposit.
Nickel or copper are common choices for electroforming. The considerable experience with electroforming nickel, especially with methods of depositing it with low internal stress, and its relatively high strength, make it the usual material of choice for electroforming.
Nickel electroforming has been performed with a wide variety of plating baths, including those containing nickel salts of sulfate and chloride (Watt's type), fluoborate, and sulfamate. The nickel sulfamate bath, first used around 1950, has become more and more the usual choice for electroforming due to its relatively low stress deposits. A nickel sulfamate bath, however, only reduces the internal tensile stress. The prior art has developed a number of additives to nickel sulfamate baths that further reduce the internal stress and, in some cases, reverse the stress to a compressive one. Some specific additive stress reducers are saccharin, naphthalenetrisulfonic acid and paratoluene sulphonamide. Small amounts of these stress reducers appear to become incorporated in the deposit. The prior art uses primarily proprietary derivatives of these additive stress reducers.
Other factors affect internal stress in the deposit including, for example, current density, temperature, nickel concentration and agitation. For given conditions of these other electrodeposition factors, there is a critical concentration of additive stress reducers that will provide zero internal stress. Unfortunately, the plating, or electroforming, bath is not completely stable in this regard due to slight decomposition of the stress reducer, its incorporation into the deposit, introduction of other minor impurities into the deposit and the general difficulty of precisely controlling deposition conditions.
The prior art has discovered that, at a combination of electroforming factors producing a nearly zero internal stress in a developing electroform, slight variations in one factor, or parameter, can proportionally move the internal stress from tensile to compressive and back again. It has been particularly found that slight changes in current from the anode to the cathodic mandrel, or master, will move the internal stress in a generally proportional direction around the zero stress point.
Measurement of internal stress during electroformation is difficult because most mandrels are made rigid to resist deformation from internal stress, thereby preventing any measurable deformation in the developing electrodeposit. The prior art has provided deformable devices to measure internal stress in a secondary electroform as an analog to the internal stress in a primary electroform on a rigid mandrel. An example is a Brenner-Senderoff spiral contractometer. It uses a flat strip of metal formed into a spiral coil. One side of the coil is painted or otherwise treated so that a deposit will form only on the outside of the coil. Internal stresses in the deposit tighten or loosen the coil to turn a rod at the coil axis which is connected to a gear and dial to magnify the rotation of the rod and produce a reading proportional to internal stress in the deposit. Typically, the prior art has used the Brenner-Senderoff spiral contractometer only to take periodic readings of internal stress for manually adjusting the electroforming bath parameters.
U.S. Pat. No. 4,648,944 to George et al describes the use of a complex strain gage assembly immersed inside the electroforming bath as a second cathode for receiving a secondary electrodeposit. A separate power supply provides current for forming the secondary deposit. The strain gage indicates any internal stress in the secondary deposit and supplies an output to a programmed micro-computer which proportionally controls the power supply for the primary electrodeposit to return the internal stress in the primary electrodeposit to zero. George et al uses a very complicated strain gage assembly and microcomputer program to achieve a near zero internal stress electrodeposited electroform. Further, the stress measured by the strain gage assembly may not be an exact analog to the stress in the developing primary electroform.
It is seen, therefore, that there is a need for an improved and simpler electroforming apparatus and method that produces deposits with near zero internal stress.
It is, therefore, a principal object of the present invention to provide an improved and simpler electroforming apparatus and method for measuring the internal stress of an electrodeposit and dynamically adjusting electrodeposition parameters to maintain the internal stress near zero.
It is another object of the present invention to provide an improved apparatus and method for electroforming deposits with near zero internal stress that provides a more even distribution of current density over the developing electroform deposit.
It is a feature of the present invention that its strain gage assembly provides an improved correspondence of measured internal stress with actual stress in the primary deposit.